When you eat spicy food, your mouth feels hot. But spiciness is not a taste. It is a kind of pain. Chili peppers contain capsaicin. It turns on a sensor in your mouth that usually feels heat above about 43°C. So your brain thinks, "Hot!"

The scientist who found this sensor won a Nobel Prize in 2021. Birds do not seem to feel chili peppers as hot, though.

Spiciness is not one of the five basic tastes, which are sweet, salty, sour, bitter and umami. Instead, it is detected by nerves that sense pain and temperature. In 1997, Julius and his team in the United States discovered the gene for a sensor that responds to capsaicin, the substance that makes chili peppers hot.

This sensor, called TRPV1, is also switched on by heat above about 43°C. When capsaicin opens it, the nerve sends a "hot and painful" signal to the brain, even though the food is not actually that hot. This work helped Julius win the 2021 Nobel Prize. Similarly, menthol in mint switches on a sensor for cold.

Although we speak of spicy "flavors," spiciness is not among the five basic tastes. It is detected not by taste cells but by nerves that sense pain and temperature. The molecular explanation came in 1997, when Julius and colleagues identified the receptor that responds to capsaicin, the compound that gives chili peppers their heat.

This receptor, TRPV1, is an ion channel on nerve endings that also opens at temperatures above about 43°C. Capsaicin therefore acts like a duplicate key. It opens the heat sensor even when nothing is hot, and the brain interprets the signal as burning pain, which may also explain why spicy food makes us sweat. This discovery contributed to Julius receiving the 2021 Nobel Prize in Physiology or Medicine.

Menthol works in a similar way on a cold receptor called TRPM8. In birds, TRPV1 is largely insensitive to capsaicin. Some researchers propose that chili plants evolved spiciness to discourage mammals, which crush seeds, while allowing birds, which swallow seeds whole and spread them, to eat the fruit. This evolutionary explanation is still a hypothesis.